Thermocouples are commonly utilized to measure the temperatures of molten materials such as glass. The glass industry basically utilizes two types of thermocouples, a ceramic enclosed thermocouple or a platinum-rhodium enclosed thermocouple. The ceramic thermocouples have good resistance to high temperatures, but have low resistance to molten glass corrosion. While the ceramic thermocouples are relatively inexpensive, their short life span can make it expensive to use this type of thermocouple over an extended period of time. The platinum-rhodium thermocouples have good resistance to high temperatures and high corrosion resistance to molten glass. However, the platinum-rhodium thermocouples are expensive due to the cost of the metals.
To measure the temperatures of a glass melt during the glass manufacturing process, a thermocouple is placed into the molten glass. Over a period of time the molten glass will corrode the material enclosing the thermocouple and will come into contact with the thermocouple junctions, resulting in the impairment of the thermoelectric circuitry which leads to the production of erroneous signals. U.S. Pat. No. 3,580,078 discloses a thermocouple failure indicating device comprising an alarm circuit located next to the thermocouple wires within a protective sheath. When the molten material corrodes the protective sheath, the molten material fills the interior of the sheath which contains crushed ceramic insulation. The insulation becomes saturated with the molten glass which results in the electrical connection of the alarm wires causing the activation of the alarm circuit which produces an electrical output. The electrical output notifies the operator of the defective condition of the thermocouple so that it can be replaced. By the time the alarm circuit of the thermocouple disclosed in the '078 patent is activated, the thermocouple is fully damaged and simple repair, if any, is not possible.
It is also known in the art to provide additional protection to refractory materials which come into contact with molten glass by coating or enclosing the refractory material with a metal or metal alloy which is resistant to the corrosive effects of molten glass. U.S. Pat. No. 3,436,203 discloses coating a refractory material with two layers. The first applied layer is an electroconductive metal oxide film. Over the first layer a thin coating or foil of a noble metal or noble metal alloy, such as platinum or platinum-rhodium alloy, is deposited, preferably bv flame spraying. Flame spraying is disclosed as being preferable to the known processes of plating or cladding since flame spraying allegedly provides better bonding to the ceramic material. According to the patent, plating and cladding can develop pinholes or minute openings through which the molten glass can contact the refractory material. U.S. Pat. No. 4,428,686 discloses a pyrometric rod containing a thermocouple. The body of the rod is made of refractory material and is inserted into a short protective tip having the shape of a glove finger. The protective tip consists of a metal or metal alloy which is resistant to molten glass, such as platinum alloyed with 10% rhodium.
The art does not disclose a thermocouple having an alarm which is activated prior to the irreparable damage or malfunction of the thermocouple.